Limits on the Macho content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds

Aims: The EROS-2 project was designed to test the hypothesis that massive compact halo objects (the so-called “machos”) could be a major component of the dark matter halo of the Milky Way galaxy. To this end, EROS-2 monitored over 6.7 years 33×10⁶ stars in the Magellanic clouds for microlensing events caused by such objects.
Methods: In this work, we use only a subsample of 7×10⁶ bright stars spread over 84 deg² of the LMC and 9 deg² of the SMC. The strategy of using only bright stars helps to discriminate against background events due to variable stars and allows a simple determination of the effects of source confusion (blending). The use of a large solid angle makes the survey relatively insensitive to effects that could make the optical depth strongly direction dependent.
Results: Using this sample of bright stars, only one candidate event was found, whereas ~39 events would have been expected if the Halo were entirely populated by objects of mass M∼0.4~M_⊙. Combined with the results of EROS-1, this implies that the optical depth toward the Large Magellanic Cloud (LMC) due to such lenses is τ<0.36×10^-7 (95% CL), corresponding to a fraction of the halo mass of less than 8%. This optical depth is considerably less than that measured by the MACHO collaboration in the central region of the LMC. More generally, machos in the mass range 0.6×10^-7M_⊙<M<15~M_⊙ are ruled out as the primary occupants of the Milky Way Halo.

Based on observations made with the Marly telescope at the European Southern Observatory, La Silla, Chile. Appendix A is only available in electronic form at http://www.aanda.org